artifact: core/router/src/main/kotlin/com/correx/core/router/model/RouterState.kt
purpose: Define RouterState as a serializable data class holding L2 memory, workflow status, and conversation history — the foundational domain type for all router projections.
depends_on: task-005
non_goals: Implementing reducer logic, defining config types
done_when: RouterState compiles as @Serializable with sensible empty defaults and is accessible from all dependent modules